SIL, BIL and your DH are all taking the piss.
If my brother and family had to come and stay beyond a couple of days, I would not expect my SIL to get my kids ready, do homework with them and do all housework, shopping and cooking.
Those of you giving the OP a hard time, would you totally abdicate responsibility for your children and expect your sahm SIL to do everything just because you're helping them out?
Thought not. Even if you're doing a favour, it would be pretty crass to expect all that and I think most people here are a little more reasonable than that. I work flexible hours so if I don't have someone doing that for me, I do it myself normally. And whilst it would be great to have someone do the school runs, the rest I'd definitely be doing my bit and not expect my guests to feel obligated to provide nanny and maid services just because we're putting them up for a few nights.
And for those saying just double the recipe. Really? That simple? Imagine having a family of 4 over for dinner or lunch. Imagine the prepping and planning to have something that 8 of you can eat at the same time, something that young kids and grown ups all want to eat. Now imagine doing that for a few nights.
I'd find it a hassle if I wasn't getting any help from any of the grown ups and I'm pretty adept at having guests for long periods -being expats, people come and stay for quite long periods.
But, OP, this is as much about your DH chipping in as your SIL.
As for the heating, stick it on. I was in the UK this week and it was fucking freezing even with the heating on and made me feel like staying in bed all day!